using remote sensing data to quantify changes in

USING REMOTE SENSING DATA TO QUANTIFY CHANGES IN VEGETATION OVER PEATLAND AREAS N. MacBean1,3,4, M. Disney1,3, P. Lewis1,3, P.Ineson2,3,4 1

Department of Geography, University College London, Gower Street, London WC1E 6BT 2 Department of Biology, University of York, York, YO10 5YW 3 NERC’s Centre for Terrestrial Carbon Dynamics 4 UK Population Biology Network ABSTRACT

A time-series of MODIS Terra daily 500m resolution surface reflectance data is investigated to examine the effect of current land management activity taking place on an upland peat site in North Wales. A catchment-scale controlled experiment has been set up to monitor the effect blocking of the artificial drainage channels (‘grips’) has on the ecosystem dynamics. The reflectance data and the Normalised Difference Vegetation Index (NDVI) and Normalised Difference Water Index (NDWI) were compared both before and after the period of grip blocking and between the control and treated catchmensts. No change was observed as a result of the grip blocking. Reasons why this might be the case are discussed. 1.

INTRODUCTION

Peatlands are widespread areas of accumulated organic matter that occur beneath a living plant layer as a result of the waterlogged nature of the soil restricting complete decay of the biomass [1]. Peatlands are important ecosystems; while only covering 3% of the land surface, they store between one-third and one one-half of the global pool of soil carbon [2]. Peat-covered landscapes, and particularly their hydrology, are highly sensitive to changes in land management, climate and pollution [3]. Many have suffered degradation due to afforestation, encroachment by alien species, over-grazing, artificial drainage, and either deliberate or accidental burning, (www[1]), resulting in erosion and loss of ecological biodiversity [4]. It is thought that such damage is causing peatlands to be converted from net sinks to net sources of carbon [2]. Therefore understanding the impact of changing management, land use and climate on peatlands is of great importance. In many moorland areas of the UK projects aimed at restoring peatlands to their ‘natural’ state are currently underway. These efforts include mowing of heather to create firebreaks and blocking of artificial drainage channels (‘grips’) (see Fig. 1). The grips were originally put in place to reduce the hazard to livestock caused by surface flooding

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owing to the typically waterlogged nature of the peat [5]. However, studies have shown that the drainage resulted in poor water quality, erosion, ecosystem destruction and flooding downstream [4]. Millions of pounds are being spent on grip-blocking and other restoration activities, and yet the precise effect on the carbon balance of the ecosystems is unknown. Earth Observation (EO) data have been widely used in ecological applications, such as for the characterization of type and percentage of land cover, derivation of biophysical variables from reflectance data such as Leaf Area Index (LAI) and estimation of canopy structure, all of which are used to calibrate, validate and drive earth system models [6]. Though field-based measurements of these ecological parameters can be made, EO offers the unique opportunity to derive the same information on a regional to global scale and at a higher temporal frequency than is realistically possible on the ground. Reflectance data gives useful information on the characteristics and dynamics of vegetation. This is due to the different ways radiation at varying wavelengths interacts with each component of the canopy. Monitoring changes in reflectance from earth observation data due to the restoration work described above will therefore enable parameterisation of carbon-balance models which will subsequently aid future policy making for the upland regions of the UK. 2. STUDY SITE AND SAMPLE DATA The study area is a fifty square kilometre upland peat site located to the west of Lake Vyrnwy in North Wales (52o 56’N, 3o 36’W) on a RSPB reserve. It is part of the Berwyn Special Area of Conservation (SAC) which has received a grant for 5 years from the EU-Life fund to restore the active blanket bog in the area (www[1]). Vegetation predominantly consists of Sphagnum mosses, heather Calluna vulgaris, heath Erica tetralix, deer grass Trichophorum cespitosum, and cotton grasses Eriophorum spp.. The main focus of restoration activity in the area is the blocking of the grips of which there are 91km in total. This is achieved by mowing 2m strips of heather to create natural

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dams, which are then placed in the grips at intervals of a few metres. An additional ~95ha of heather will be mown to create firebreaks. The project started in 2006 with each phase taking place over the winter months due to work being prohibited during the bird nesting season.

NDVI =

A catchment-scale experiment has been set up to monitor the effects of the grip blocking. This provides a unique opportunity to explore the impact of large-scale maniplulations with control. Five catchments have been separated into ‘control’ and ‘treated’ areas. Grip blocking will only take place in the treated areas until the end of the scientific experiments, in order to examine the effect of the grip blocking on the water table, carbon fluxes. NASA’s Moderate Resolution Imaging Spectrometer (MODIS) Terra’s daily surface reflectance L2G 500m and 1km v005 (MOD09GA) datasets for tile h17v03 were acquired between the dates of 1st September 2006 and 31st May 2008. The data was assessed against the quality product and only those pixels which passed were used in the study.

(1)

where NIR represents the value of reflectance in the Near Infrared (MODIS band 2), and Red the value of reflectance in the red waveband (MODIS band 1).

NDWI =

Fig. 1. Aerial photos taken from the University of Edinburgh Geosciences Aircraft on 19th March, 2008. The roughly linear artificial drainage channels can easily be distinguished against the heather. (Width on ground ~500m).

NIR
Re d NIR + Re d

NIR
SWIR NIR + SWIR

(2)

where SWIR represents the value of reflectance in the Short-Wave Infrared. NDVI is sensitive to changes in vegetation cover due to the high ratio of near-infrared to red wavelengths characteristic of a spectral profile of vegetation. NDWI is sensitive to changes in the leaf water content of the vegetation as water is a good absorber of midinfrared energy [7]. NDWI is used as the grip blocking is expected to cause a decrease in water table depth, with subsequent increase in soil moisture. This may result in an increase in plant moisture content which would result in a decreased NDWI. 4. RESULTS Overall there is no observable change in reflectance after the grip blocking has taken place in any band (fig 2 and 3).

3. METHODOLOGY The effect of grip blocking is examined using a time-series of MODIS surface reflectance data between 1st September 2006 and 31st May 2008. Blocking of the grips took place in an area to the south of the Eunant Fawr River between 12th December 2007 and 30th March 2008. The ‘control’ and ‘treated’ areas are compared to determine whether there is a difference between the two areas after the grip blocking has taken place. The Normalised Difference Vegetation Index (NDVI) equation (1) and the Normalised Difference Water Index (equation 2) are also used to investigate the change due to grip blocking.

Fig. 2. Surface reflectance for MODIS bands 1, 3 and 4 for before and after the grip blocking took place. The grip blocking took place between day 470 and day 600.

The reflectance values after the time period where the grip blocking took place are generally higher, but are no different to the same time period the previous year, suggesting the change seen immediately before and after the grip blocking took place is due to the natural variability of the ecosystem.

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Fig. 3. Surface reflectance for MODIS bands 2, 5, 6 and 7 for before and after the grip blocking took place. The grip blocking took place between day 470 and day 600.

The reflectance in the control area is generally higher than in the treated area (fig 4). However this is the case both before and after grip blocking has occurred, and as there is a similar density of grips in both areas, this pattern is therefore probably due to slightly different dynamics in the control and treated areas than a difference due to the grip blocking.

Fig. 4. Comparison of the reflectance in the control and treated areas both before and after blocking has occurred, plotted as a function of day of year between September to August of the following year for MODIS bands 2, 3 and 6 respectively, (bands used to calculate the vegetation and water indices). Other bands showed similar patterns.

Similarly neither the calculated NDVI (fig. 5) or NDWI (fig. 6) indices showed any variation after the grip blocking had taken place than the same time period the previous year before the grips had been blocked.

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Fig. 5. Comparison of the NDVI of the control and treated areas before and after grip blocking, plotted as a function of day of year from September to August of the following year

Fig. 6. Comparison of the NDWI of the control and treated areas before and after grip blocking, plotted as a function of day of year from September to August of the following year.

5. CONCLUSION The change in reflectance and vegetation and water indices in the months following the time period of grip blocking can is not apparent in the time series of daily MODIS surface reflectance data presented here. This may be due to the fact that no change in reflectance occurs due to this management activity, that the change is much more subtle or occurs over a longer time period that this dataset, or that the resolution of the images is too coarse to be able to identify changes which happen in a relatively small proportion of the pixel. A study of the underlying dynamics of the ecosystem needs to be carried out. The data presented in this study suggests there might be a natural difference between the control and treated areas which would mean using the control area cannot be used to compare with any change detected in the treated area. A longer time series of reflectance data needs to be used to determine over whether a change due to the blocking of the grips can be observed over a longer time period. We will also obtain monthly ground based measurements of reflectance taken along transects across the blocked and unblocked grips, in addition to having four permanent NDVI sensors located in two separate control and treated catchments. Further work will also focus on understanding how grip blocking affects the soil and plant dynamics, and whether this will result in a change in the reflectance for each wavelength. Particularly, a detailed analysis of the relationship between the water table, soil moisture content, leaf water content and the resulting reflectance signal for

each different vegetation type represented in the study area will be completed. A range of ecosystem models will also be calibrated using field measurements, and used to predict the carbon fluxes from the peat system under different hydrological conditions. The aim of the modelling work is to firstly estimate the magnitude of the system carbon fluxes under the changing management; and secondly to allow assimilation of reflectance data to constrain the model estimates. ~In this way it is envisaged that an optimal estimate carbon budget of the system will be obtained, allowing both the baseline of, and changes to the system due top management to be better quantified. Higher resolution Landsat and IKONOS scenes as well as airborne measurements of reflectance will also be studied for signs of change due to grip blocking to address the hypothesis that the change may be too localised to detect in the coarse MODIS data. 6. ACKNOWLEDGEMENTS This work was carried out as part of a PhD funded by the Natural Environment Research Council through the Centre for Terrestrial Carbon Dynamics, with additional funds provided through the UK Population Biology Network. 7. REFERENCES [1] D. Charman, Peatlands and Environmental Change, John Wiley & Sons, Ltd, England, 2002. [2] J. Holden, “Peatland hydrology and carbon release: why smallscale process matters”, Philosophical Transactions of the Royal Society A, The Royal Society, London, pp. 2891-2913, 2005. [3] O.M. Bragg and J.H. Tallis, “The sensitivity of peat-covered upland landscapes”, Catena, Elsevier Science B.V., pp. 345360, 2001. [4] J. Holden, P.J. Chapman and J.C. Labadz, “Artificial drainage of peatlands: hydrological and hydrochemical process and wetland restoration”, Progress in Physical Geography, Arnold, London, pp. 95-123, 2004. [5] M Haigh, “Environmental Change in Headwater Peatlands, UK”, Environmental Role of Wetlands in Headwaters, Springer, Netherlands, 2006. [6] S. Liang, “Recent developments in estimating land surface biogeophysical variables from optical remote sensing”, Progress in Physical Geography, Sage Publications, pp. 501516, 2007. [7] J.R. Jenson, “Remote Sensing of the Environment, An Earth Resource Perspective”, Prentice Hall, 2000. www[1] http://www.blanketbogswales.org/

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